Solve for $y$ : $-7 = 8 + y$
Subtract $8$ from both sides: $ \begin{eqnarray} \\ \hphantom{-7 {- 8}} &\hphantom{=}& \hphantom{y} \\ -7 &=& 8 + y \\ \\ {-8} && {-8} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -7 {- 8} &=& y \\ \end{eqnarray} $ Simplify. $ y = -15$